How does it work?

Well, first of all it is a httpModule (IHttpModule) hooked into the HttpApplication.EndRequest event. On the EndRequest event it simply checks to see if there is a .js file that matches either the file extension, file name or file path of the current request.

For example:
A request to http://www.mydomain.com/myfolder/mypage.aspx would initiate a check for the following .js files…
- /jsInject/Extensions/.aspx.js
- /jsInject/FileNames/mypage.aspx.js
- /jsInject/Paths/myfolder/mypage.aspx.js

If the corresponding .js file exists it will be injected it into the request.

Check out the full blog post here and download the source and binaries here